1. Field of the Invention
The present invention relates to a light scanning apparatus for deflecting a laser beam so that the laser beam scans a photosensitive member, and an image forming apparatus having the light scanning apparatus.
2. Description of the Related Art
In recent years, digital copying machines and digital printers have rapidly become widespread, and various types of image forming apparatus have been put into practical use.
A so-called laser beam printer, which is one image forming apparatus using an electrophotographic process, modulates a laser beam output from a semiconductor laser according to an image signal and deflects the laser beam by a rotary polygon mirror. The laser beam deflected by the rotary polygon mirror scans a photosensitive member of a photosensitive drum or the like. When the laser beam scans the photosensitive drum, an electrostatic latent image is formed on the photosensitive drum. The latent image is developed into a toner image by a developing device. The toner image on the photosensitive drum is transferred onto a sheet via an intermediate transfer member, such as an intermediate transfer belt. The transferred toner image on the sheet is then fixed by heat treatment. In this way, the image formation is performed.
Now, a light scanning apparatus (a laser beam scanning system) of a laser beam printer will be described. FIG. 14 illustrates the schematic structure of the laser beam scanning system of the laser beam printer.
The laser beam scanning system of the laser beam printer includes a semiconductor laser 1 for emitting a laser beam as a light beam, a rotary polygon mirror 3, and an fθ lens 4, and forms an electrostatic latent image on a photosensitive drum 5 by emitting the laser beam to the photosensitive drum 5 from the semiconductor laser according to image data. A laser beam LB emitted from the semiconductor laser 1 passes through a cylindrical lens 2 to reach the rotary polygon mirror 3. The rotary polygon mirror 3 is rotationally driven by a driving motor (not shown). The semiconductor laser 1 is supplied with a PWM signal, which has been subjected to pulse width modulation based on an image clock and an image signal, so that the semiconductor laser 1 emits the laser beam LB according to the PWM signal. The laser beam LB having been emitted from the semiconductor laser 1 to reach the rotary polygon mirror 3 is deflected by the rotary polygon mirror 3 and converted by the fθ lens 4 so that a moving speed (scanning speed) of the laser beam on the photosensitive drum may be constant. The laser beam LB therefore scans the surface of the photosensitive drum 5 at a constant speed in the direction indicated by the arrow A of FIG. 14. Part of the laser beam having passed through the fθ lens 4 is reflected by a BD reflecting mirror 6, which is disposed at a position that does not irradiate a region of the photosensitive drum 5 in which an image is to be formed (hereinafter, referred to as an image region). The laser beam is then received by a BD sensor 7. In the image region, the laser beam LB is reflected by a reflecting mirror 8 after passing through the fθ lens 4 and irradiates the photosensitive drum 5. The photosensitive drum 5 is charged by a charging device (not shown), and an electrostatic latent image is thus formed when the photosensitive drum is irradiated with the laser beam LB.
Subsequently, a speed control method for the driving motor in the laser beam scanning system will be described with reference to FIG. 15. FIG. 15 is a block diagram of a control circuit, schematically adding the driving motor 9. The BD reflecting mirror 6 for detecting a BD signal is not illustrated.
Referring to FIG. 15, the BD signal as a main scanning synchronization signal is input to a frequency division circuit 10. The frequency division circuit 10 divides a frequency of the BD signal by a value equal to the number of mirror surfaces of the rotary polygon mirror 3. In FIG. 15, the number of the mirror surfaces is “8”.
Meanwhile, the reason why the frequency of the BD signal is divided by the number 8 of the mirror surfaces of the rotary polygon mirror 3 will be briefly described with reference to FIG. 16. The rotary polygon mirror 3 is not a complete regular polygon because of limited manufacturing accuracy. Accordingly, even when the rotation of the driving motor 9 is stable, the cycle of the BD signal fluctuates as indicated by T1 to T8 of FIG. 16. Therefore, if the driving motor 9 is controlled based on this BD signal, appropriate control cannot be performed.
On the other hand, when the frequency of the BD signal is divided by 8 to generate a BD/8 signal, as represented by the waveform of the BD/8 signal of FIG. 16, the BD signal is shaped into a signal having a single pulse for each revolution of the driving motor 9. In this case, if a rotary speed of the driving motor 9 is stable at a target speed, the BD/8 signal is not affected by fluctuations in surface shape of the rotary polygon mirror 3, and the cycle thereof is always constant (Tround). In this way, the rotary cycle of the driving motor 9 can be measured accurately without being affected by the fluctuations in surface shape of the rotary polygon mirror 3. This is the reason why the BD signal is divided by the number of the mirror surfaces of the rotary polygon mirror 3.
Returning to FIG. 15, the BD signal whose frequency has been divided by the frequency division circuit 10 (i.e., BD/8 signal) is input to a counter 11. The counter 11 measures the cycle of the BD/8 signal, that is, the rotary cycle of the driving motor 9.
The count value Tround of the counter 11 and a target cycle Ttarget stored in a target value storing portion 12 are input to a control signal generating portion 13. The control signal generating portion 13 calculates an acceleration control amount and a deceleration control amount of the driving motor 9 based on the count value Tround and the target cycle Ttarget, and generates an ACC signal (acceleration signal) and a DEC signal (deceleration signal). Note that, as the target cycle Ttarget, a target cycle corresponding to the BD/8 signal is set.
A motor driving portion 20 generates a motor driving signal according to the ACC signal and the DEC signal input from the control signal generating portion 13, to thereby rotationally drive the driving motor 9.
In the laser beam printer, the laser beam is scanned as described above to form an electrostatic latent image on the photosensitive drum.
Actually, however, the scanning speed of the laser beam on the photosensitive drum fluctuates a little every scanning, and the formed electrostatic latent image therefore suffers from magnification fluctuations in the laser beam scanning direction, which thus degrades image quality.
One factor causing the fluctuations in scanning speed of the laser beam is fluctuations in rotary speed of the rotary polygon mirror.
As described above, the rotary polygon mirror is under speed control through the measurement of its cycle for every revolution and is accelerated or decelerated even during image formation. The fluctuations in rotary speed are thus present within a control range, resulting in the fluctuations in scanning speed of the laser beam. Such fluctuations are present even if the shape of the rotary polygon mirror is an ideal regular polygon.
To address such fluctuations, Japanese Patent Application Laid-Open No. H10-148773 proposes a method in which the rotary speed of the rotary polygon mirror is detected to correct a write timing of the laser beam based on the result of detection so as to reduce image misregistration caused by the fluctuations in rotary speed of the rotary polygon mirror.
Further, another factor causing the fluctuations in scanning speed of the laser beam is the accuracy of the surface shape of the rotary polygon mirror. The rotary polygon mirror is desired to be an ideal regular polygon, but actually has a manufacturing error. When the mirror surface shows a convex tendency, the scanning speed of the laser beam is slower than that measured when the mirror surface is an ideal flat surface, thus resulting in a small scanning magnification. When the mirror surface shows a concave tendency, the scanning speed of the laser beam is faster, thus resulting in a large scanning magnification. Such fluctuations in scanning speed are present even if the rotary polygon mirror ideally rotates at a constant speed, and periodically occur along with the rotation of the rotary polygon mirror.
A solution to such fluctuations is proposed in Japanese Patent Application Laid-Open No. H03-110512. Japanese Patent Application Laid-Open No. H03-110512 proposes a method in which a scanning magnification of each mirror surface is measured in advance to correct a write timing of the laser beam at the time of scanning on the each mirror surface by utilizing the result of measurement so as to reduce image misregistration caused by the fluctuations in surface shape.
The above-mentioned related art, however, has the following problems.
As described above, in the laser beam scanning system of a laser beam printer, the rotary polygon mirror is accelerated or decelerated even during image formation so as to be controlled to have a rotary speed fall within a given range. In other words, specifically, the rotary speed of the rotary polygon mirror fluctuates with the acceleration or deceleration, with the result that fluctuations in magnification in an electrostatic latent image occur.
The method proposed in Japanese Patent Application Laid-Open No. H10-148773 can eliminate long-period fluctuations in rotary speed of the rotary polygon mirror, but cannot eliminate the above-mentioned short-period fluctuations occurring in association with acceleration or deceleration of the rotary polygon mirror. As described above, the speed control of the rotary polygon mirror is performed by detecting the rotary cycle of the rotary polygon mirror every revolution. However, the fluctuations in scanning speed in association with acceleration or deceleration of the rotary polygon mirror are short-period fluctuations such that the scanning speed varies every scanning made by the rotary polygon mirror.
For example, in Japanese Patent Application Laid-Open No. H10-148773, the rotary speed is detected every revolution of the rotary polygon mirror to correct the write timing, which cannot eliminate the above-mentioned short-period fluctuations, thus resulting in a control error.
Further, in Japanese Patent Application Laid-Open No. H03-110512, in order to eliminate the magnification fluctuations caused by the accuracy of the shape of the rotary polygon mirror, the magnification of each mirror surface is measured in advance to correct the write timing. In Japanese Patent Application Laid-Open No. H03-110512, however, the above-mentioned fluctuations in rotary speed of the rotary polygon mirror occurring in association with the acceleration or deceleration are not taken into consideration. In reality, the magnification fluctuations among the mirror surfaces show a difference between when the rotary polygon mirror is accelerated and decelerated, thus also resulting in a control error.